In a wireless communication system, a transmitter transmits wireless signals to a receiver through a physical channel, such as air, in the form of electromagnetic waves. Due to the non-ideal channel effect, such as multi-path reflection and multi-path fading, the wireless signals received by the receiver may get distorted.
Based on the orthogonal frequency division multiplexing (OFDM) modulation technology for the multi-carrier modulation, the effective processing ability is obtained with respect to the multi-path reflection effect. In the OFDM systems, the receiver only needs one simple one-tap equalizer to equalize the frequency selective fading effect, which is caused by time invariant multi-path channel. So, the OFDM system has recently become the mainstream technology in the communication field and the broadcasting application development.
For the previous wireless communication system, multi-path signals are assumed to be useless even harmful. However, characteristics of multi-path propagating are efficiently utilized to improve throughput, transmission distance/coverage and reliability in a multi-transmitting multi-receiving antenna system applying a multi-input multi-output (MIMO) technique. Thus, the MIMO technique has recently become another popular technology.
Therefore, a new MIMO OFDM system is developed. The MIMO OFDM system can be classified into techniques capable of improving the transmission distance/coverage and reliability, such as a space time block code (STBC) technique, and techniques capable of improving the data transmission throughput, such as a Bell Labs Layered Space-Time (BLAST) technique. So, the MIMO OFDM system has recently become the mainstream technology in the wireless communication application development. Various systems, such as the wireless local area network (WLAN) 802.11n, the European standard digital video broadcasting-terrestrial 2 (DVB-T2), and the Wi-Max IEEE 802.16e/802.16m and the 3rd generation partnership project (3GPP) long term evolution (LTE)/long term evolution-advanced (LTE-A) with the mobility apparatus, uses the MIMO OFDM transmission technology.
Nowadays, receivers of some MIMO OFDM systems (e.g., WLAN 802.11n, DVB-T2, IEEE 802.16e/802.16m, 3GPP LTE/LTE-A and the like) are emphasized to provide desirable receiving capability at the high-speed motion. However, the receiver of the MIMO OFDM system will suffer problems as described below when the STBC technique is applied. A received signal from each receiving antenna is a composition signal of all transmitted signals from the transmitting antennas as the MIMO technique is applied. The demodulation of one of the received signals will be interfered with the transmitted signals from all the other transmitting antennas, thereby causing the co-channel interference (CCI). The above problem is usually overcome by the orthogonal characteristic of the STBC coding technique. However, when the receiver is moved at the high speed relatively to the transmitter, the fast variation of the channel will enhance the CCI, so that the orthogonal characteristic the STBC coding technique provided is no longer kept, thereby causing the detection of the received signals easily making mistakes and the performance of the system degraded.
In addition, when the receiver of the MIMO OFDM system is not stationary and is moved at the high speed relatively to the transmitter, the channel in the duration of one OFDM useful symbol is no longer kept in the fixed state, thereby causing the time-selective fading channel. Due to the influence of the Doppler effect at the high-speed motion, the OFDM signal is positively or negatively offset by one time of Doppler frequency (fd) with the center carrier frequency (fc) serving as the center. This offset is unfavorable to a multi-carrier modulation system, and may cause an inter-carrier interference (ICI) effect that destroys the orthogonality of the sub-carriers, hence resulting in an error floor phenomenon in the performance of bit error rate (BER).
A CCI cancellation method is disclosed in U.S. Pat. No. 7,403,571 B2. A receiver estimates channel responses of different transmitting antennas and receiving antennas at different time, and calculates gains of channel responses. Thereafter, the receiver selects a received signal with a better channel response gain to perform data detection, and then utilizes the detected data to reconstruct and cancel the CCI term of the received signal with a worse channel response gain to perform data detection for the other transmitted data. Since the CCI term is reconstructed by the detected data, the insufficient reliability of the detected data will degrade the performance of the CCI cancellation method. Besides, neglect of the reconstruction and cancellation of the ICI term will also degrade the overall system performance.
Proc. of IEEE GLOBECOM, PP. 1-5, 2009, discloses a CCI and ICI reconstruction and cancellation method. In this article, a recursive manner is provided to cooperate with the CCI and ICI interference cancellation method to suppress the CCI and ICI effect of the STBC OFDM system. The method presets a number of recursion at the receiver, estimates channel responses of different transmitting antennas and receiving antennas at different time, and then calculates gains of channel responses. Thereafter, the receiver selects the received signal with a better channel response gain to perform data detection, and then utilizes the detected data to reconstruct and cancel the CCI term of the received signal with a worse channel response gain to perform data detection for the other transmitted data. Then, the receiver determines whether the number of recursion is exceeded. If the number of recursion is not exceeded, the reconstruction and cancellation of the ICI term of the received signal is performed, and the reconstruction and cancellation of the CCI term is repeatedly according to the above steps until the preset number of recursion is achieved. Although the method considers the ICI terms and enhances the reliability of data detection by the recursive manner, the recursive manner may cause the complexity of overall computation to be raised and the operation processing speed to be slower.
A zero-forcing equalizer is provided in U.S. Pat. No. 7,483,364 B2. A receiver estimates channel responses of different transmitting antennas and receiving antennas at different time, and forms channel response matrices corresponding to the received signals. Then, the zero-forcing equalization technique is utilized to perform an inverse matrix operation to compensate channel effect, including the influence of CCI. However, assume the number of the transmitting antennas is M, the zero-forcing equalizer needs to perform the inverse matrix operation with the matrix size of M×M, and O(M3) complex multiplication operations are needed. As the value of M is very large, the complexity of hardware implementation is too high.